SAN FRANCISCO—New research suggests the distinct oxygenation events that created Earth’s breathable atmosphere happened spontaneously, rather than being a consequence of biological or tectonic revolutions.

The study, presented today at AGU’s Fall Meeting and published in the journal Science, not only shines a light on the history of oxygen on our planet, but also gives new insight into the prevalence of oxygenated worlds other than our own.

The early Earth had no oxygen in its atmosphere or oceans until roughly 2.4 billion years ago when the first of three major oxygenation events occurred. The reasons for these stepwise increases of oxygen on Earth have been the subject of ongoing scientific debate.

In the new research, scientists modified a well-established conceptual model of marine biogeochemistry so that it could be run over the whole of Earth history and found that it produced the three oxygenation events all by itself.

“The model demonstrates that a gradual oxygenation of Earth’s surface over time should result in distinct oxygenation events in the atmosphere and oceans, comparable to those seen in the geological record,” said author Benjamin Mills, who will present results from the new study today at the Fall Meeting at a press conference live-streamed at 1:30 p.m. PST. Mills leads the biogeochemical modelling group at University of Leeds.

The new findings suggest that beyond early photosynthetic microbes and the initiation of plate tectonics—both of which were established by around three billion years ago—it was simply a matter of time before oxygen would reach the necessary level to support complex life.

This new theory drastically increases the possibility of high-oxygen worlds existing elsewhere.

“This research really tests our understanding of how the Earth became oxygen rich, and thus became able to support intelligent life,” said the study’s lead author Lewis Alcott, a geochemist at the University of Leeds. “Based on this work, it seems that oxygenated planets may be much more common than previously thought, because they do not require multiple – and very unlikely – biological advances, or chance happenings of tectonics.”

The first Great Oxidation Event occurred during the Paleoproterozoic era—roughly 2.4 billion years ago. The subsequent wholesale oxygenation events occurred in the Neoproterozoic era around 800 million years ago and finally in the Paleozoic Era roughly 450 million years ago, when atmospheric oxygen rose to present day levels.

Large animals with high energy demands require high levels of oxygen, and evolved soon after the last of these steps, ultimately evolving into dinosaurs and mammals.

Currently, the two prevailing theories suggest the drivers of these oxygenation events were either major steps in biological revolutions—where the evolution of progressively more complex lifeforms essentially bioengineere” oxygenation to higher levels—or tectonic revolutions—where oxygen rose due to shifts in the style of volcanism or make-up of the crust.

The new study instead highlights a set of feedbacks that exist between the global phosphorus, carbon and oxygen cycles, which are capable of driving rapid shifts in ocean and atmospheric oxygen levels without requiring any stepwise change in either tectonics or biology.

“Our model suggests that oxygenation of the Earth to a level that can sustain complex life was inevitable, once the microbes that produce oxygen had evolved,” said author Professor Simon Poulton, also from the School of Earth and Environment at Leeds.

Their Earth system model of the feedbacks reproduces the observed three-step oxygenation pattern when driven solely by a gradual shift from reducing to oxidizing surface conditions over time. The transitions are driven by the way the marine phosphorus cycle responds to changing oxygen levels, and how this impacts photosynthesis, which requires phosphorus.

“Our work shows that the relationship between the global phosphorus, carbon and oxygen cycles is fundamental to understanding the oxygenation history of the Earth. This could help us to better understand how a planet other than our own may become habitable,” Mills said.

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Notes for journalists

Author Benjamin Mills will present these findings at a press conference at the AGU Fall Meeting on Tuesday, 10 December at 1:30 p.m. PST in room 310-312 on level three of Moscone Center South, 747 Howard St, San Francisco, CA 94103.

This press conference will be streamed live on the AGU press events webpage. Reporters can visit this site throughout the meeting to watch press conferences in real time and ask questions via an online chat. Press conference recordings will be archived on AGU’s YouTube channel. For the full press conference schedule visit the Fall Meeting 2019 Media Center

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